Connected component
Connected component is highly connected subgraph of the graph. Thus, every graph may be divided to several connected components.
Find connected components
In order to find connected components you have to choose Algorithms -> Find connected components in the menu bar. You will see the result immediately. Distinct connected components will be marked with distinct colours and will have a unique number.
Algorithm
Graph Online finds connected component for non-oriented graphs and it finds weakly connected component for oriented graphs. Breadth-first search is used for search. Implementation on JavaScript could be seen here: http://graphonline.ru/script/plugins/ConnectedComponent.js